1. Field of the Invention
The present invention relates to sensors for detecting a substance in liquid using a surface acoustic wave element (SAW element), and more specifically, to a sensor for detecting a substance in liquid, the sensor including a sensing SAW element and a reference SAW element.
2. Description of the Related Art
Various sensors have been developed for detecting a substance to be detected (detection-target substance) in liquid, for example, a protein. For example, WO2006/027893A1 listed below discloses a sensor for detecting a substance in liquid using a surface acoustic wave element. FIG. 10A is a plan view of a sensor for detecting a substance in liquid described in WO2006/027893A1, and FIG. 10B is a front cross-sectional view that illustrates a main portion thereof.
A sensor 101 for detecting a substance in liquid includes a base substrate 102. The base substrate 102 includes an upper surface 102a in which recesses 102c and 102d are provided at locations spaced from a first end 102b. A sensing SAW element 104 and a reference SAW element 105 are disposed in the recesses 102c and 102d, respectively. A resin layer 103 having holes 103b and 103c facing the recesses 102c and 102d, respectively, is laminated on the base substrate 102.
The sensing SAW element 104 includes a piezoelectric substrate, an interdigital transducer (IDT) electrode provided on the piezoelectric substrate, and a reaction film arranged so as to cover the IDT electrode. The reaction film is made of a material that reacts with a detection-target substance in liquid and couples to the detection-target substance. The reference SAW element 105 has a configuration in which an IDT electrode is provided on a piezoelectric substrate, and the reference SAW element 105 has no reaction film.
In use, at least the portions in which the holes 103b and 103c are disposed are placed in liquid, and the reaction film of the sensing SAW element 104 reacts with and couples to a detection-target substance. Accordingly, in the sensing SAW element 104, a mass on the portion in which the IDT electrode is disposed is increased by the coupling to the detection-target substance. In contrast, in the reference SAW element 105, because the reaction film that reacts with the detection-target substance is not provided, there is no mass increase caused by the coupling to the detection-target substance.
In the sensor 101 for detecting a substance in liquid described in WO2006/027893A1, a change in the speed of sound of a surface-acoustic wave caused by the addition of mass in the sensing SAW element 104 is detected as a change in an electrical signal. In this case, the detection-target substance is detectable with high precision by determining the difference between an output from the sensing SAW element and an output from the reference SAW element.
A similar sensor for detecting a substance in liquid is also disclosed in WO2006/027945A1.
As described above, the sensor 101 for detecting a substance in liquid detects the presence or absence, and the density of a protein in liquid using the difference between an output signal from the sensing SAW element 104 and that from the reference SAW element 105. In this case, specifically, the difference between an oscillation frequency of an oscillation circuit including the sensing SAW element and that of an oscillation circuit including the reference SAW element is determined to detect the presence or absence or the density of the detection-target substance.
It is desirable for the sensor 101 that, when the detection-target substance is not present, the characteristic of the sensing SAW element 104 and that of the reference SAW element 105 be approximately equal.
However, when the detection-target substance comes into contact with the SAW elements 104 and 105, the difference between the oscillation frequency of the oscillation circuit including the sensing SAW element 104 and that including the reference SAW element 105 may be relatively small. In such a case, both oscillations may be electromagnetically coupled, and both of the oscillation frequencies may be substantially the same. For this reason, it is difficult to detect the detection-target substance in liquid reliably with high precision. Even when both oscillations are not coupled, measurement sensitivity may be relatively small depending on the detection-target substance, and the detection-target substance may be undetectable.